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A Little on the Top, for Environment's Sake

If you want a greener urban environment, start at the top. The top of a building, that is, where a cover of vegetation — a green roof, as it were — can help reduce energy bills and improve air quality.

Green roofs have become popular in Europe, and they are beginning to catch on in North America. Some use ground-cover plants like sedum and other succulents (a roof at Ford's River Rouge complex in Michigan is an example) while others are more like gardens, with shrubs and even trees.

Green roofs can provide immediate benefits, but what about the long term? Scientists at the University of Toronto investigated the environmental impact of a green roof over the expected life of a building. They modeled the impact over 50 years on an eight-story apartment building, designed by Susana Saiz, an architect who is one of the researchers. The building, in Madrid, currently has a conventional flat roof, although it could support a green one.

Christopher Kennedy, a professor of civil engineering at the university and an author with Ms. Saiz and others of a paper describing the work, said that such environmental "life cycle assessments" evaluate elements like the cost of construction materials and maintenance as well as energy use.

For instance, Dr. Kennedy said, a green roof may use materials that are produced with less of an impact on the environment, and may protect the standard roofing material beneath it so it does not have to be replaced as often.

But he said the main effect comes from energy savings. Vegetation absorbs less sunlight than a conventional dark roof, and some of the energy that is absorbed is used in evapotranspiration from the plants. So a green roof stays cooler, and less energy is required to cool the living space beneath it.

The paper, published this month in Environmental Science and Technology, showed a 6 percent energy savings in summer and 1 percent overall. The Madrid building has a relatively small roof area; the savings would be greater with a wider, lower building.

If green roofs catch on in a neighborhood, the impact may be compounded by reducing the "heat island" effect — the scorching heat off roofs and streets that raises temperatures. "If you can get a one-degree reduction in temperature in a city in the summer, it's a huge savings in energy you use for cooling," Dr. Kennedy said.

Stirring Up Soil Trouble

Much hay has been made over the effect of some modern agricultural practices on soil. Nutrients are lost, soil structure breaks down, and the soil itself is lost to erosion.

But modern farmers aren't the only ones who have problems keeping the soil in good shape. Research on the island of Maui suggests that the Polynesians who settled Hawaii had trouble, too.

Photo

Credit
Chris Gash

Anthony Hartshorn of the University of California, Santa Barbara, and colleagues looked at the effect on soil nutrients of cultivation of taro and sweet potato on the slopes of Haleakala volcano. As the Polynesian settlers expanded into the drier, leeward parts of the islands beginning in the 1400's, they brought these crops with them.

Archaeological evidence shows that in these drier regions, the crops were grown on a portion of volcanic slopes where conditions were right. These slopes had a zone of highly stratified soil about three feet thick over lava bedrock, consisting of a top layer of ash, an intermediate layer of larger nutrient-rich cinders and another ash layer that served as a water reservoir on the bottom.

To reach this bottom layer to quench their crops' thirst, the early Hawaiians dug through the surface ash and cinder layers with a stick, rotating it to create a cone-shaped hole for planting. That way the roots could reach the water below.

But the researchers report this week in The Proceedings of the National Academy of Sciences that this practice resulted in the depletion of calcium, magnesium and other nutrients over time. Their experiments showed that digging crushed some of the cinders, increasing their surface area, and also mixed cinders with moist ash. Both conditions led to increased leaching of nutrients from the cinders.

The researchers also estimated that crop yields were probably sufficient over a long period — perhaps 300 years. Eventually, however, the depletion of soil nutrients would have led to lower yields of these crops.

Warmer Times Down South

Scientists have discovered the remnants of elephant seal colonies in Antarctica near the Ross Ice Shelf, implying that the region's climate was warmer then than it is today. The finding may also have some implications for the future of the ice shelf itself.

Breeding grounds of southern elephant seals currently are found well north of Antarctica, mostly on remote islands in the Southern Ocean. The animals need access to open water, and the extent of pack ice around the continent is now too great.

But Brenda L. Hall of the University of Maine and colleagues found seal skin, hair and other remains at several sites along the coast of Victoria Land on the Ross Sea. The finding, published last week in The Proceedings of the National Academy of Sciences, demonstrates that seals used to breed and molt there. They suggest that warming caused the pack ice to thin or disappear.

Radiocarbon analysis dated the remains from about 250 years ago to more than 6,000 years ago. In particular, the seals flourished from 1,100 to 2,300 years ago, indicating a previously unrecognized period of unusual warmth in the region.

Many scientists are concerned that as the climate continues to warm, huge expanses of ice like the Ross Ice Shelf will thin and eventually disintegrate, even within this century. But if the new study is correct, the Ross shelf apparently survived very warm conditions more than 1,000 years ago.

A Supercomputer Goes Silent

Supercomputers come and go, but ASCI Red, a supercomputer used to simulate nuclear weapons tests at Sandia National Laboratory, has had staying power. The computer, developed by Intel and consisting of more than 9,000 processors that operate in parallel, was the first to exceed a trillion mathematical operations per second. It was ranked the fastest in the world seven times in a three-year period ending in 2000. Even now it's in the top 500.

All good things must pass, however, and ASCI Red has now been retired. It proved extremely reliable, but it costs a lot to house it and keep it cool. Newer computer technology is more energy-efficient.